The present invention relates to a mold for use in producing a lens and, more particularly, to a mold suitable in producing a lens having a concave surface.
Conventionally, a method of molding a lens is known which is arranged as follows. Specifically, a glass material and a mold are heated to temperature at which the glass material is softened, that is, to temperature corresponding to glass viscosity of 10.sup.7.5 to 10.sup.11 poises. The glass material is press-molded by the use of a pair of upper and lower or first and second mold portions of the mold, to form a lens. Subsequently, the lens is cooled to temperature in the vicinity of glass transition temperature that is 10.sup.13 poises, or to a temperature lower than the glass transition temperature. Then, the pair of first and second mold portions are released from the lens or are moved away from each other, and the lens is taken out or removed from the pair of first and second mold portions.
As shown in FIG. 11 of the attached drawings, in the case where a lens 3 having its both convex surfaces is molded on the basis of the above-described molding method, a shrinkage difference between the lens 3 and a pair of upper and lower or first and second mold portions 1 and 2 of a mold 7 causes gaps 4 to occur between the lens 3 and the first mold portion 1 and between the lens 3 and the second mold portion 2, at the cooling step from the press molding temperature to the mold-release temperature. Accordingly, air enters locations between the mold portions 1 and 2 and the lens 3 through the gaps 4, so that it is possible to easily release the lens 3 from the mold portions 1 and 2. Generally, the mold 7 has its coefficient of thermal expansion which is less than that of the lens 3.
In the case where a lens having its one or both concave surfaces, however, the following disadvantage or problem occurs. That is, as shown in FIG. 12, the shrinkage difference between the glass material of the lens 3 and the mold 7 at cooling causes a closed gap space 5 to occur between the mold portion 1 and the lens 3. Because the gap space 5 is negative in pressure, the lens 3 tends to adhere to the mold portion 1. Thus, it is difficult to release the lens 3 from the mold 7. In this case, if a physical force is applied to the lens 3 to release the same from the mold 7, a surface accuracy of the lens 3 is damaged.